1. Field of the Invention
The present invention relates to a semiconductor device which constitutes the photodetector part of a switching device used in combination with a light-emitting diode (LED).
2. Description of the Prior Art
In recent years, with the achievement of rapid development in semiconductor technology, there have been increased demands for higher performances and miniaturization of hardware in various control systems, along with digitalization thereof. However, the input/output parts of these control systems still process analog signals, and when viewed from the whole system, digital circuits and analog circuits are arranged in a mixed form within the same system. Accordingly, it is an important subject that analog signals (particularly, minute voltages and currents) are reliably processed and that the processed signals are introduced into the control part of the system.
As a device for controlling an output-side circuit insulated electrically from an input-side circuit according to the signals given to the input-side circuit, electromagnetic relays have been mainly used. However, electromagnetic relays have movable mechanical parts; therefore the relay itself is large in size, and it is difficult to make the equipment using this relay smaller in size. Moreover, electromagnetic relays have disadvantages in that the movable mechanical parts are easily fatigued and the life time thereof is short.
In place of such electromagnetic relays, semiconductor relays, referred to as solid state relays (SSRs), having the advantages of small size, light weight, and a long life time have been widely applied. For example, an optically coupled semiconductor relay has been developed, which is composed of a light emitting part, comprising a light emitting diode, and a photodetector part, comprising a photodiode array and a metal-oxide semiconductor field effect transistor (MOSFET).
As a switching device of this kind, there is used, for example, an optically coupled semiconductor relay such as shown in FIG. 5. In such an optically semiconductor relay, light emitted from a forward-biased LED 40 is received by a photodiode array 41 which produces a voltage higher than the threshold voltage when exposed to light radiation, the produced voltage is applied to the gate G of a normally-off n-channel MOSFET 42 to cause conduction to take place between the drain D and source S thereof, and the positive charge stored at the gate G of the MOSFET 42 is released through the photodiode array 41 which loses the photoelectromotive force upon the darkening of the LED, to cut off conduction between the drain D and source S.
However, the release of the positive charge through the photodiode array 41, as mentioned above, takes considerable time because of the large resistance of the array, resulting in a longer fall time when switching off the MOSFET 42, and hence, a drop in the switching characteristics. For improvement, a technique may be considered in which the positive charge is released through a low-resistance resistor 43 inserted in parallel with the photodiode array 41, as shown by a dotted line in FIG. 5. This, however, will in turn cause the problem that the photoelectromotive force generated in the photodiode array 41 by the illumination from the LED becomes shorted, which prevents the gate G voltage of the MOSFET 42 from reaching the threshold value, thus resulting in insufficient conduction.
To solve the above problem, a technique has previously been proposed in which a discharging MOSFET is used, as shown in FIGS. 6 and 7. In the circuit shown in FIG. 6, a normally-on n-channel MOSFET 44 is connected between the gate G and source S of the switching MOSFET 42, while between the gate G and drain D of the MOSFET 44, a resistor 45 and an additional photodiode array 46 in the reverse direction are connected in parallel with each other. When the MOSFET 42 remains conducting by the action of the photodiode array 41 exposed to light radiation from the LED 40, the photodiode array 46 applies a voltage higher than the negative threshold voltage to the gate G of the discharging MOSFET 44 to cut off conduction between the source S and drain D thereof. On the other hand, when the LED 40 goes off, the positive charge stored at the gate G of the MOSFET 42 is rapidly released through the discharging MOSFET 44 which conducts upon the extinction of the electromotive force in the photodiode array 41, thus quickly putting the switching MOSFET 42 into the nonconducting state.
On the other hand, in the circuit shown in FIG. 7, a normally-on p-channel MOSFET 47 is connected between the gate G and source S of the MOSFET 42, while a resistor 48 is connected between the gate G and drain D of the MOSFET 47, and a diode 49 in the forward direction is connected between the gate G and source S thereof. When the MOSFET 42 remains conducting by the photoelectromotive force of the photodiode array 41, the MOSFET 47 to the gate G of which a positive voltage is applied by the photoelectromotive force is put in the nonconducting state, and the positive charge stored at the gate G of the MOSFET 42 is rapidly released through the discharging MOSFET 47 which conducts upon the extinction of the photoelectromotive force of the photodiode array 41, thus quickly putting the switching MOSFET 42 into the nonconducting state.
However, the circuit shown in FIG. 6 has the disadvantages of increasing the size and cost of the optically coupled semiconductor relay because it requires the additional photodiode array 46 connected in the reverse direction between the gate G and drain D of the normally-on n-channel discharging MOSFET 44 in order to operate the MOSFET 44 in reverse to the normally-off n-channel switching MOSFET 42. Also, it is usual to form the photodiode array and the discharging MOSFET, and sometimes, the switching MOSFET also, on a single chip for compactness in size. In such a case, since both MOSFETs 42 and 44 in the circuit of FIG. 6 are n-channel devices with n-type source and drain regions formed in a p-type substrate, there will be no problem if the photodiode array 41 is formed on the p-type substrate with a n-type photodetector layer deposited thereon. On the other hand, in the case of the circuit of FIG. 7 which uses the MOSFETs 42 and 47 of different channel types from each other, the former being n-type and the latter p-type, if the photodiode array 41 is formed in the same type as in FIG. 6, there will be no need for an additional photodiode array because it has the reverse construction to that of the p-channel MOSFET 47 having its p-type source and drain formed in a n-type substrate, but this circuit construction has the disadvantage in that it involves increased numbers of processes and masks used during the manufacture of the semiconductor chip.